Annealing (metallurgy)

Annealing, in metallurgy and materials science, is a heat treatment wherein a material is altered, causing changes in its properties such as strength and hardness. It is a process that produces conditions by heating and maintaining a suitable temperature, and then cooling. Annealing is used to induce softness, relieve internal stresses, refine the structure and improve cold working properties.

In the cases of copper, steel, and brass this process is performed by substantially heating the material (generally until glowing) for a while and allowing it to cool slowly. In this fashion the metal is softened and prepared for further work such as shaping, stamping, or forming.

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Stages of annealing

There are three stages in the annealing process, with the first being the recovery phase, which results in softening of the metal through removal of crystal defects (the primary type of which is the linear defect called a dislocation) and the internal stresses which they cause. The second phase is recrystallization, where new grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has been completed, grain growth will occur, in which the microstructure starts to coarsen and may cause the metal to have less than satisfactory mechanical properties.

Annealing in a controlled atmosphere

The low temperature of annealing (about 50 °F above C3 line) may result in oxidation of the metal’s surface, resulting in scale. If scale is to be avoided, annealing is carried out in an oxygen-, carbon-, and nitrogen-free atmosphere (to avoid oxidation, carburization, and nitriding respectively) such as endothermic gas (a mixture of carbon monoxide, hydrogen gas, and nitrogen).

The magnetic properties of mu-metal (Espey cores) are introduced by annealing the alloy in a hydrogen atmosphere.

Diffusion annealing of semiconductors

In the semiconductor industry, silicon wafers are annealed, so that dopant atoms, usually boron, phosphorus or arsenic, can be incorporated into substitutional positions in the crystal lattice, resulting in drastic changes in the electrical properties of the semiconducting material.

Thermodynamics of annealing

Annealing occurs by the diffusion of atoms within a solid material, so that the material progresses towards its equilibrium state. Heat is needed to increase the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in metals and (to a lesser extent) in ceramics. This alteration in dislocations allows metals to reform more easily, so increases their ductility.

The amount of process-initiating Gibbs free energy in a deformed metal is also reduced by the annealing process. In practice and industry, this reduction of Gibbs free energy is termed "stress relief".

The relief of internal stresses is a thermodynamically spontaneous process; however, at room temperatures, it is a very slow process. The high temperatures at which the annealing process occurs serve to accelerate this process.

The reaction facilitating the return of the cold-worked metal to its stress-free state has many reaction pathways, mostly involving the elimination of lattice vacancy gradients within the body of the metal. The creation of lattice vacancies are governed by the Arrhenius equation, and the migration/diffusion of lattice vacancies are governed by Fick’s laws of diffusion.

Mechanical properties, such as hardness and ductility, change as dislocations are eliminated and the metal's crystal lattice is altered. On heating at specific temperature and cooling it is possible to bring the atom at the right lattice site and new grain growth can improve the mechanical properties.

Specialized annealing cycles

Normalization

Normalization is an annealing process in which a metal is cooled in air after heating.

This process is typically confined to hardenable steel. It is used to refine grains which have been deformed through cold work, and can improve ductility and toughness of the steel. It involves heating the steel to just above its upper critical point. It is soaked for a short period then allowed to cool in air. Small grains are formed which give a much harder and tougher metal with normal tensile strength and not the maximum softness achieved by annealing.

Full anneal

A full anneal typically results in the softest state a metal can assume. To perform a full anneal, a metal is heated to its annealing point and held for some time to allow the material to fully austenitize. The material is then allowed to cool slowly so that the equillibrium microstructure is obtained. In some cases this means the material is allowed to air cool. In other cases the material is allowed to furnace cool. The details of the process depend on the type of metal and the precise alloy involved. In any case the result is a more ductile material that has greater elongation and reduction of area properties but a lower yield and tensile strength. This process is also called LP annealing for lamellar pearlite in the steel industry as opposed to a process anneal which does not care about the microstructure and only has the goal of softening the material. Often material that is annealed will be machined and then be followed by further heat treatment to obtain the final desired properties.

See also

• Tempering, which is very similar to stress relieving and post-weld heat treat (PWHT) processes
• Annealing (glass)
• Heat treatment
• Hollomon-Jaffe parameter